Erythrocytes glutathione (GSH) levels were measured in erythrocytes from 33 subjects, at baseline and after 2-hour glucose loading in order to investigate the effect of glucose ingestion on the erythrocyte GSH. According to the World Health Organisation criteria 18 subjects had normal glucose tolerance (NGT)(mean age 48 +/- 10 years, 10 women, 8 men), 15 subjects had impaired glucose tolerance (IGT)(mean age 52 +/- 8 years, 9 women, 6 men). After 12-hour fasting, erythrocyte GSH levels were 40.5 +/- 8.06 and 39.27 +/- 10.26 mg/dl hemolisate in subjects with NGT and IGT, respectively (p = N.S). After 2-hour glucose loading, erythrocyte GSH levels decreased to 36.01 +/- 9.4 (p < 0.05) and 32.36 +/- 5.7 (p < 0.005) in subjects with NGT and IGT, respectively. The decrease in erythrocyte GSH levels in subjects with IGT was greater than in NGT individuals (p < 0.001). There was negative correlation between glucose, insulin, C-peptide, and erythrocyte GSH levels after glucose loading (p < 0.005). Our results suggest that glucose loading induce an oxidative stress in all subjects but this oxidative stress is greater in subjects with IGT than with NGT.